1. Field of the Invention
This invention relates to a sucked substrate detecting apparatus, and more particularly to an apparatus for transferring substrates by use of a vacuum suction technique.
2. Description of the Prior Art
In recent years, a compact disk (hereinafter, referred to as a CD) has been widely used to record a large amount of digitized sound and image information. The substrate of the CD is generally made of transparent synthetic resin (polycarbonate and the like). A large number of pits are formed on the substrate surface in accordance with the digitized information ("1" or "0") to be recorded. Further, an aluminum thin film layer having a high light-reflectivity is formed on the substrate surface by use of a sputtering technique. The digital information recorded by the arrangement of the pits can be read by recognizing the presence or absence of the reflected light of a laser light beam applied to the CD.
The aluminum thin film can be formed by sputtering on the surface of a single substrate in a relatively short time. Thus, a continuous sputtering system has been used in which the aluminum thin film can be continuously formed on a large number of substrates. In the continuous sputtering system, a vacuum suction transferring apparatus has been employed which can minimize damage to the substrates in the processes of transferring the substrates to be sputtered into a sputtering apparatus from the outside of the continuous sputtering system and also processes of transferring the sputtered substrates to the outside. A conventional vacuum suction substrates to the outside. A conventional vacuum suction apparatus is shown in FIG. 4. In FIG. 4, a substrate 41 is being sucked by sucking pads 42. The sucking pads 42 are connected through a main pipe 43 and a first valve 44 to a vacuum pump 45. Further, a branch pipe 46, which communicates with the atmosphere through a second valve 47, is connected to the main pipe 43 at a portion between the sucking pads 42 and the first valve 44. The open-and-close operations of the first and second valves 44 and 47 are controlled in accordance with control signals produced from a controller 48. In this vacuum suction apparatus, when the substrate 41 is sucked by the sucking pads 42, the second valve 47 is closed to isolate the atmosphere, and the first valve 44 is opened so as to exhaust the air from the main pipe 43 by use of the vacuum pump 45. To the contrary, when the substrate 41 is released from the sucking pads 42, the first valve 44 is closed, and then the second valve 47 is opened so as to communicate with the atmosphere.
In the continuous sputtering system, it is important to confirm whether or not a large number of substrates are being smoothly transferred, i.e., to confirm whether or not the substrate 41 is being satisfactorily sucked by the sucking pads 42. A conventional substrate detecting apparatus comprises a light generator 49 and a reflected light detector 50 which are provided in the vicinity of the sucking pads 42, as shown in FIG. 4. A light beam 51 produced from the light generator 49 is applied to the surface of the substrate 41 being captured by the sucking pads 42. A light beam 52 reflected from the surface of the substrate 41 is detected by the reflected light detector 50. As a result, the substrate 41 being captured by the sucking pads 42 can be confirmed. The above-described substrate detecting apparatus is of a light reflecting type utilizing a light reflecting film (made of aluminum and the like) formed on surface of the substrate 41. Besides this, there is a light penetrating-type substrate detecting apparatus in which the sucked substrate can be recognized by the presence or absence of the penetrating light. However, both the types of the substrate detecting apparatus can merely detect whether or not the substrate 41 has been captured by the sucking pads 42 and physically exists.
Therefore, in the above-described conventional substrate detecting apparatus, the detection of the physical existence of the substrate 41 on the sucking pads 42 does not necessarily indicate that the substrate 41 has been satisfactorily sucked by the sucking pads 42. For example, even when a foreign substance has been interposed between the substrate 41 and the sucking pads 42, the conventional substrate detecting apparatus detects this as if the substrate 41 had been satisfactorily sucked by the sucking pads 42. Thus, the substrate 41, which has been unsatisfactorily sucked by the sucking pads 42, is transferred without any adjustement into the subsequent transferring process. As a result, the substrate 41 might fall off the sucking pads 42 or might be dislocated. This prevents the maintenance of the normal film-formation processes, and causes the failure in the system. Further, in general, the continuous sputtering film-formation is successively performed with an operation period of about 6 seconds. This means that the damage caused by the unsatisfactory substrate-sucking state is not limited to the corresponding substrate alone, but also to the entire operations of the continuous sputtering system. Such disadvantages are not limited to the single-CD processing system.